Owing to the incipient changes in the electricity market as well as those which are also to be expected in the near future, cost-optimized and interruption-free generation and distribution of electrical power is desirable for power supply organizations. Generators are of particular importance in this context, whose failure results in high financial costs, so that there is a growing interest in improving the operational reliability.
Motors, generators and the like typically have a rotor which is mounted in a stator core such as it can rotate. The stator core has stator windings which are composed of insulated bundles of conductors, which are known as stator bars and are embedded in slots in the stator core.
The high-voltage conductors are generally insulated. The insulation which surrounds such high-voltage conductors deteriorates over time. Deterioration in the insulation and in the characteristics of the insulation can result in partial discharge activity within the insulation. This discharge activity causes further deterioration in the characteristics of the insulation. Damage can occur as a result of locally limited breakdowns resulting from field strength peaks in the insulating medium. In the long term, these breakdowns damage the insulation. Increasing deterioration of the insulation leads to increasing partial discharge activity, which in turn speeds up the deterioration of the insulation.
An insulated conductor which has been subject to partial discharge activity may need to be replaced in order to prevent or to rectify a fault or failure in the stator winding system. In this case, the motor or generator must be shut down and must be disassembled. This is a costly and time-consuming process. It is thus advantageous to have the capability to determine the state of the insulation in the stator windings in advance in order to predict whether and approximately when repair will become necessary, so that the repair can be carried out in a ordered and well-organized manner, before the fault or the malfunction becomes evident, and at a time which is most suitable with respect to the operating plan of the specific motor or generator which is affected.
Partial discharge activities can be detected by various methods, in particular by chemical, acoustic or electrical methods. In the case of physically extended conductors, such as those which are used in generators and motors, partial discharge pulses are highly deformed on their path from the point of origin to the measurement point, are attenuated and have reflected partial discharge signals as well as external interference signals superimposed on them, so that the partial discharge source can be located, and the partial discharges and interference signals can be separated only in rare cases and with major measurement and computation complexity.
The detection and assessment of partial discharges on physically extended arrangements such as generators or similar high-voltage appliances is frequently associated with major difficulties because of the deformation of the partial discharge signals as a result of the characteristic attenuation properties and the superimposition of external interference signals.
One method for locating partial discharges is described in the article “Ein neuartiges Sensorsystem zur Erfassung von Teilentladungen an gieβharzisolierten Transformatoren” [A novel sensor system for detecting partial discharges in transformers with cast resin insulation], Peter Werle, Volker Wasserberg, Hossein Borsi, Ernst Gockenbach, Schering Institute for High Voltage Technology and High-Voltage Installations, Hanover University, Germany. According to this document, sensors which are designed to detect partial discharge signals are distributed at equidistant intervals on a conductor. A partial discharge propagates from its point of origin in both directions of the conductor bar. Those sensors which are arranged closest to the point of origin of the partial discharge detect the strongest signal. In this case, the location of a partial discharge can be determined by means of an evaluation unit which is connected to all of the sensors. This method has the disadvantage that, rather than determining the precise location of the partial discharge, it determines a region between two sensors as the possible point of origin of a partial discharge.
A further method for locating partial discharges in transformers and similar high-voltage appliances is described in DE 100 05 540 A1. In this case, the partial discharge source can be located based on a system theory by convolution of a transfer function and partial discharge signals measured at external terminals. This is done by determining the best match between the back-calculated input signals and the partial discharge signal that is produced at the true point of origin.
DE 689 22 727 T4 discloses a further method and an apparatus for partial discharge detection. In this case, a sensor is used to detect a signal which is caused by a partial discharge. The point of origin of a partial discharge is located with a relatively high error probability by analysis of the signal.
Further methods and apparatuses for partial discharge identification are described in DE 197 58 087 A1 and in DE 199 62 834 A1.